Precious metal scavenging from a liquid medium with a functionalised polymer fiber

ABSTRACT

A method for the recovery of a metal from a liquid medium containing the metal I solution or in finely divided insoluble from comprises contacting the medium with a functionalized polymer fiber capable of binding the metal and recovering the metal from the fiber. The polymer fiber is suitably a polyolefin, a fluorinated polyethylene, cellulose or viscose, which is functionalized by the radiation grafting of at least one monomer. The method is particularly suitable for the recovery of platinum group metals from process residues.

This application is the U.S. national phase application of PCTInternational Application No. PCT/GB01/04540.

This invention relates to a method for the scavenging of metals fromliquid media, more particularly to a method for the recovery of platinumgroup metals (PGMs) from organic, aqueous or mixed organic/aqueoussolutions.

The widespread use of noble metals such as PGMs as either heterogeneousor homogeneous catalysts for chemical processes generates substantialamounts of waste solutions or streams of various compositions. Theeconomical use of catalysts based on PGMs is almost always dependent onthe efficient recovery of the catalyst, whether by recycling thecatalysts themselves, or by the efficient recovery and refining of thenoble metal. As is known in the art, the PGMs comprise the lower membersof group VIII of the periodic table namely, platinum, palladium,rhodium, iridium, ruthenium and osmium.

Heterogeneous catalysts, in which the noble metal is anchored to a solidsupport, are often easy to recover by filtration. Loss of metal ismainly due to the loss of fine particulates during filtration or due tosolubilisation of the noble metal in the reaction media. The noblemetals are usually recovered by incineration and/or leaching proceduresand the noble metal is worked up in a conventional manner.

However, the recovery of homogeneous catalysts is not straightforward.If the reaction solution, including reactants, product(s) and solvent,is low boiling and composed only of noble metal compounds, the metal canbe concentrated using distillation and the catalysts can possibly bereused several times. If the solution contains other non-noble metalinorganic compounds, salts or high boiling solvents, a useful way ofrecovery is to add the solution to the smelting process of a noble metalmelt. Other ways of treating organic solutions include combustion andpyrolysis however, such processes may give rise to air pollution,especially when phosphorous is present in the work up solutions.Furthermore, losses of precious metals in any pyrometallurgical processcan be high as can processing costs, including capital and energy costs.

Processes based on precipitation of the noble metals have beendeveloped. These are based for example, on the recovery of the noblemetal by precipitation with elemental sulphur, or sulphur compounds(U.S. Pat. No. 4,273,578), or with elemental tellurium or reducibletellurium compounds (U.S. Pat. No. 4,687,514).

EP 0429017 A1 describes a process to remove rhodium containing catalystsfrom a solution of hydrogenated nitrile rubber, by passing the residuethrough an ion-exchange column containing a macroreticular resinmodified with a selective amine, thiol, carbodithioate, thiourea and/ordithiocarbamate functional group. From the comparative examplespresented in the patent it is clear that non-macroreticular resins, i.e.gel type resins, are unsuccessful in removing rhodium from organicsolutions.

The invention described in U.S. Pat. No. 4,388,279 is concerned with aprocess for the recovery of trace amounts of noble metals which havebeen used as catalysts in organic reactions. Products resulting fromsuch reactions are contacted with solid adsorbents selected from GroupIA and Group IIA of the Periodic Table, molecular sieves and an ionexchange resins. Examples are given of the performance of calciumcarbonate for the recovery of rhodium, but no data are given on theperformance of ion exchange resins.

U.S. Pat. No. 5,208,194 describes a process for recovering Group VIIItransition metal carbonyl complexes by contacting an organic solutionwith an acidic ion exchange resin containing sulfonic acid groups.Preferred resins are macroreticular or macroporous resins having surfaceareas greater than about 20 m²/g. According to the patent, stronglybasic, weakly basic, neutral, and weakly acidic ion exchange resins areunsuitable for use.

EP 0355837 A2 describes a method for recovery of a transition metal frompolar or non-polar liquids, by contacting the liquid with anion-exchange resin having bonded ionically thereto an organophosphorousligand. The ligand is ion-exchanged onto traditional ion-exchange resinsand the metal to be recovered forms a complex with the ligand.

EP 0227893 A2 describes a method for the removal of dissolved metalsfrom solutions using a microporous ethylene polymer with pendantcarboxylic acid groups. Comparative examples are described which showthat equivalent non-porous materials are not effective. The porosity ofthe polymer is therefore crucial to the effectiveness of the processdescribed. Furthermore, the polymer does not have equal affinity forsimilar metals for example, the affinity is higher for Pd, Ir and Ruthan it is for Os, Re and Pt.

Hence, according to the state of the art, macroreticular and porousresins are preferred over gel-type ion-exchangers for recovery ofprecious metals from organic solutions. However, based on the patentliterature, the recoveries obtained with macroreticular resins areinadequate to allow them to be used commercially in organic solutions.There are also many further problems attached to the use ofmacroreticular resins in metal scavenging applications from organicsolutions. The mechanical stability of porous polymers is often notsufficient to withstand use in stirred reactors without creating fines.Osmotic stability is an even bigger problem since the loading of ahomogeneous PGM complex with attached ligands gives a very high weightincrease of the material inducing an osmotic shock that disintegratesthe polymer and blocks pores. The porous structure also results indifficulties in further processing of the resins by for example,elution. During elution the material is transferred to an aqueous phase.The treated organic solutions are often viscous and difficult to removefrom the porous material. Organic materials will block the pores of theresin and this material is poorly removed during the elution of theresin. Gel-type materials would hence be preferred. However, traditionalgel-type resins function poorly in organic solution mainly due to thelarge dimensions of the beads, and due to the crosslinks introduced tothe materials during preparation of the resins.

It is an object of the present invention to develop materials andmethods for the easy, efficient and economical recovery of metals fromorganic solutions. It has now been found that ion-exchange groupsattached to fibrous materials show excellent metal binding propertiesfrom various organic-based residues, solutions and streams.

In accordance with the present invention a method for the recovery of ametal from a liquid medium containing the metal in solution or in finelydivided insoluble form comprises contacting the medium with afunctionalised polymer fibre capable of binding the metal; andrecovering the metal from the fibre.

The present invention has application to organic, aqueous and mixedorganic/aqueous media containing metals in metallic or other insolubleform or, preferably, in solution. Such media may be process or effluentstreams, or may be streams from the refining of metals, especially therefining of PGMs. The preferred media are those in which a single PGM ispresent in solution in an organic solvent or a mixed organic/aqueoussolvent. Desirably, in the latter case, the organic solvent is misciblewith the aqueous system. Some examples of mixed organic/aqueous mediainclude dimethylformamide/water mixtures, alcohol/water mixtures, wherethe alcohol may be any liquid alcohol, or acetonitrile/HCl mixtures.Aqueous systems include salt or acid solutions.

The metal may be from any group of the periodic table for example, atransition metal, an alkali or alkaline earth metal such as Ca, a heavymetal, or a rare earth metal. Desirably, the metal comprises atransition metal, or a heavy metal such as Hg, Pb or Cd. The transitionmetal may be noble metal or a base metal active as a catalyst orcatalyst promoter such as Ni, Co or W. Most preferably, the metalcomprises a noble metal, especially one or more of the PGMs.

Preferably, the method further comprises the addition of a precipitatingor complexing agent to yield a form of the metal having improved bindingcharacteristics for the functionalised polymer fibre. Suitableprecipitating or complexing agents include those selected from the groupof thiourea, urea, amines and polyamines.

Preferably, the polymer is substantially non-porous. The lack ofporosity provides the polymers with sufficient mechanical strength towithstand use in stirred reactors without creating fines. Difficultiesassociated with further processing of the polymers by for example,elution are also mitigated.

Preferably, the polymer fibre comprises a polymer chosen from the group;polyolefins, fluorinated polyethylene, cellulose and viscose.

Suitable polyolefins are those formed from units of α-olefins, the unitshaving the formula —CH₂—CHR—, where R is H or (CH₂)_(n)CH₃ and n is inthe range of 0 to 20. Particularly suitable polyolefins are those whichare homo- or co-polymers of ethylene and propylene. In the case offluorinated polyethylenes, those formed from units of the generalformula —F₂—CX₂—, where X is H or F are suitable. For example,polyvinylidene fluoride and polytetrafluoroethylene are particularlypreferred.

It has been shown by the present inventors that noble metals orcomplexes of noble metals can be scavenged from organic or mixedaqueous/organic media using functionalised polymer fibres, that ispolymer fibres onto which suitable functional groups have beenintroduced.

The functional groups can be introduced in various ways includingradiation grafting, chemical grafting, chemical modification ofpre-formed fibres or further chemical modification of grafted fibres,formation of interpenetrating networks etc. Preferably, the functionalgroups are introduced by radiation grafting.

Graft copolymers can be prepared in various ways but radiation graftingis an especially suitable method for graft modification of polymerfibres. Radiation grafting is generally known, and involves theirradiation of a polymer in a suitable form, for example, film, fibre,pellets, hollow fibre, membrane or non-woven fabric, to introducereactive sites (free radicals) into the polymer chain. These freeradicals can either combine to give crosslinks, as is the case forpolyethylene, or cause chain scission as is the case for polypropylene.On the other hand, the free radicals can be utilised to initiate graftcopolymerisation under specific conditions. Three different methods ofradiation grafting have been developed; 1) direct radiation grafting ofa vinyl monomer onto a polymer (mutual grafting); 2) grafting onradiation-peroxidized polymers (peroxide grafting); and 3) graftinginitiated by trapped radicals (pre-irradiation grafting).Pre-irradiation grafting is mostly preferred since this method producesonly small amounts of homopolymer in comparison to mutual grafting.

Preferably, the functionalised polymer fibre comprises at least onefunctional group selected from; carboxylic, sulphonic, pyridinium,isothiouronium, phosphonium, amine, thiol or the like, and grafted vinylmonomers such as acrylic acid, methacrylic acid, acrylates,methacrylates, styrene, substituted styrenes such as α-methyl styrene,vinyl benzyl derivatives such as vinyl benzyl chloride, vinyl benzylboronic acid and vinyl benzyl aldehyde, vinyl acetate, vinyl pyridine,and vinyl sulphonic acid.

The functionalised fibres may be added to the solution to be treated ina stirred tank or the solution to be treated may be passed through acolumn packed with the fibres. It may be advantageous to heat thesolution for example, in the range from ambient to 100° C.

In the present invention, fibres may be used without further processingand be of any length however, they have the very substantial advantageover polymer beads in that they may be converted, using conventionaltechnology, into a great variety of forms. Thus, fibres may be spun,woven, carded, needle-punched, felted or otherwise converted intothreads, ropes, nets, tows or woven or non-woven fabrics of any desiredform or structure. Fibres can easily be stirred in a liquid medium, andfiltered off or otherwise separated therefrom. If desired, fibres ofdifferent characteristics can readily be combined in threads or fabrics,in order to optimise the metal scavenging properties for a particularfeedstock medium. In an embodiment, fibres may be combined withinorganic fibres such as silica or alumina fibres in order to provideincreased mechanical strength. This may be of use when the fibres areused in processes which involve high degrees of agitation or highturbulence.

The noble metal may be recovered by filtering the fibres from thesolution and recovering the noble metal by eluting the fibres using anion-displacing reagent such as a strong acid or salt or a complexingagent, e.g. a sodium salt, or by destroying the fibre structure, e.g.using pyrolysis or hydrolysis, to produce a metal concentrate. Theconcentrate can then be worked up in a conventional manner.

The invention will now be described by way of non-limiting example only,and it will be appreciated that skilled person will readily see manyopportunities to use the present invention in all its aspects.

EXAMPLE 1

Polyethylene fibres were irradiated under inert atmosphere to total doseof 150 kGy using an electron accelerator operating at an accelerationvoltage of 175 kV and beam current of 5 mA. The irradiated fibres wereimmediately immersed in a reaction mixture containing styrene, vinylbenzyl chloride and ethanol. The reaction mixture was purged withnitrogen before initiating the reaction and the grafting reaction wasallowed to continue to completion, which usually took approximately 6hours.

The resulting fibres were filtered from the reaction solution and washedfirstly with ethanol and then with dichloroethane.

EXAMPLE 2

Polyethylene fibres were irradiated under an inert atmosphere to totaldose of 150 kGy using an electron accelerator operating at anacceleration voltage of 175 kV and beam current of 5 mA. The irradiatedfibres were immediately immersed in a reaction mixture containing4-vinyl pyridine and ethanol. The reaction mixture was purged withnitrogen before initiating the reaction and the grafting reaction wasallowed to continue to completion, which usually took approximately 6hours. The resulting fibres were filtered from the reaction solution andwashed firstly with ethanol and then with dichloroethane or with adilute aqueous acid.

EXAMPLE 3

Polyethylene fibres were irradiated under an inert atmosphere to totaldose of 150 kGy using an electron accelerator operating at anacceleration voltage of 175 kV and beam current of 5 mA. The irradiatedfibres were immediately immersed in a reaction mixture containingstyrene and ethanol. The reaction mixture was purged with nitrogenbefore initiating the reaction and the grafting reaction was allowed tocontinue to completion, which usually took approximately 6 hours. Theresulting fibres were filtered from the reaction solution and washedfirstly with ethanol and then with dichloroethane.

EXAMPLE 4

100 g fibres prepared as in Example 1 were stirred in ethanol for 1hour. 20 g of thiourea dissolved in ethanol was added and the stirringcontinued for a further 2 hours. The fibres were filtered from thesolution and washed with ethanol before further use.

EXAMPLE 5

Fibres prepared as in Example 3 were added to a solution ofdichloroethane and left overnight. Chlorosulphonic acid was added understirring and the stirring continued for 5 hours. The fibres werefiltered from the solution and treated with 2M sodium hydroxidesolution, washed with acidified water to pH 1, and finally washedrepeatedly with distilled water.

EXAMPLE 6

130 g of an oxo-ester residue containing 395 ppm of palladium wasdissolved in a dimethyl formamide/water mixture. Fibres prepared as inExample 2 were added to the solution and the dispersion stirred overnight at room temperature. The palladium content of the solutiondecreased to 40 ppm.

EXAMPLE 7

The same solution as used in Example 6 was stirred at room temperatureover night with fibres prepared as in Example 4. The palladium contentof the solution decreased to 3 ppm.

EXAMPLE 8

A glass column was packed with fibres prepared as in Example 2. The samesolution as in Example 6 was passed through the column. An ash contentof 4% by weight, analysed as Pd, was achieved and less than 3 ppm Pdremained in solution.

EXAMPLE 9

A carbonylation residue solution containing 105 ppm of rhodium wasstirred with fibres prepared as in Example 5. The rhodium content of thesolution decreased to approximately 50 ppm when the solution was boiledin presence of the fibres.

EXAMPLE 10

The same solution as in Example 9 was stirred with fibres prepared as inExample 4. The rhodium content of the solution decreased to 45 ppm whenstirred over night at 60° C. However, when thiourea in ethanol was addedto the solution and stirring continued at 60° C. for approximately 2hours the rhodium content of the solution decreased to 3 ppm.

EXAMPLE 11

Thiourea dissolved in ethanol was added to a hydroformylation residuesolution containing 850 ppm of rhodium. Fibres prepared as in Example 5were added to the solution under stirring. After 1 hour, the rhodiumcontent of the solution decreased to 20 ppm. When DMF was used insteadof ethanol, the rhodium content of the solution decreased to 10 ppmunder similar reaction conditions.

EXAMPLE 12

A high boiling point distillation residue from a coupling reactioncontaining 4.5% palladium and 4.5% phosphorus, present as a triarylphosphine, was dissolved in an ethanol/thiourea mixture under reflux.Fibres prepared as in Example 5 were added to the solution and stirredfor 60 minutes. Approximately 97-99% of the palladium was recovered onthe fibres.

EXAMPLE 13

A carbonylation residue containing 105 ppm of rhodium was used for acomparative trial of fibres versus a commercially-available strong acidcation exchanger (“Amberlyst”). This bead form ion-exchange materialcontains the same sulfonic acid functionality as the fibre prepared inExample 5. 80 ml ethanol containing 2 g thiourea was added to 200 ml ofthe carbonylation residue and heated at 60° C. for 30 minutes. To halfof this solution, 2 g of dry Amberlyst beads were added and stirred at60° C. for 2 h. To the other half of the solution, 2 g of dry fibresprepared as in Example 5 were added and stirred at 60° C. for 2 h. Therecovery of rhodium for the Amberlyst beads was found to be 43%,compared with 98.5% for the scavenging fibres according to Example 5.

EXAMPLE 14

Fibres prepared according to example 4 were further treated by stirringfor 2 hours in an ethanol solution containing 2M sodium hydroxide. Thefibres were filtered from this solution, washed with distilled water andtreated with acid to pH 1. The fibres were re-filtered and washed withdistilled water to neutral pH.

EXAMPLE 15

Fibres prepared according to example 14 were immersed in a reactionresidue from a coupling reaction which contained THF, triaryl phosphinesand 30 ppm of palladium. The fibres and residue were refluxed for 1hour, after which time no palladium was detectable in the reactionresidue.

1. A method for the recovery of a metal from a liquid medium containingthe metal in solution or in finely divided insoluble form, the methodcomprising the steps of adding a precipitating or complexing agent tothe medium, wherein the precipitating or complexing agent is selectedfrom the group consisting of at least one of thiourea, urea, amines andpolyamines; contacting the medium with a functionalised polymer fibrecapable of binding the metal; and recovering the metal from the fibre;wherein the precipitating or complexing agent yields a form of the metalhaving improved binding characteristics for the functionalised polymerfibre; and wherein the liquid medium is a liquid organic medium and themetal comprises a platinum group metal (PGM).
 2. A method according toclaim 1, wherein the medium is a process residue of stream comprisingcatalyst residues or catalyst, an effluent stream or a refining streamfrom the refining metals.
 3. A method according to claim 1, wherein thepolymer fibre comprises a polymer selected from the group consisting ofpolyolefins, fluorinated polyethylene, cellulose and viscose.
 4. Amethod according to claim 1, wherein the functionalised polymer fibrecomprises at least one functional group selected from: carboxylic,sulphonic, pyridinium, isothiouronium, phosphonium, amine, thiol,grafted vinyl monomers, acrylic acid, methacrylic acid, acrylates,methacrylates, styrene, substituted styrenes, α-methyl styrene, vinylbenzyl derivatives, vinyl benzyl chloride, vinyl benzyl boronic acid,vinyl benzyl aldehyde, vinyl acetate, vinyl pyridine, and vinylsulphonic acid.
 5. A method according to claim 4, wherein the at leastone functional group is introduced by radiation grafting.
 6. A methodaccording to claim 1, wherein the functionalised polymer fibre is spun,woven, carded, needle punched, felted or otherwise converted intothreads, ropes, nets, tows or woven or non-woven fabrics.
 7. A methodaccording to claim 6, wherein the fibre is combined with inorganicfibres.
 8. A method according to claim 1, wherein the liquid medium isheated at up to 100° C.
 9. A method according to claim 1, whereinrecovering the metal from the fibre comprises eluting with anion-displacing reagent.
 10. A method according to claim 9, wherein theion-displacing reagent is selected from the group consisting of a strongacid, a salt and a complexing agent.
 11. A method according to claim 1,wherein recovering the metal from the fibre comprises destroying thefibre by pyrolysis or hydrolysis.